This invention relates to the preparation of polyurethane-urea elastomers.
Polyurethane-urea elastomers are often prepared by reacting a relatively high equivalent weight active hydrogen-containing material and a relatively low equivalent weight active hydrogen-containing material (hereinafter "chain extender") with a polyisocyanate. In preparing the elastomer, the reactive components and any catalysts or other optional additives are blended and then transferred to a mold of suitable shape where the formulation is cured. It is typical practice to only cure the elastomer in the mold until it is capable of maintaining its shape, and then demolding the elastomer and postcuring it until the poymerization is completed. In this manner, the molds can be used more often, permitting higher production.
Since it is usually desirable to produce as many parts as possible in a given period of time, it is desirable that the residence time in the mold be as short as possible. A short residence time permits the mold to be used more often in a given time period. Accordingly, it is desirable that the elastomer formulation cure relatively rapidly in the mold to a state at which the elastomer can be demolded and postcured. Conversely, it is necessary that the formulation not cure too quickly, since some time is required to blend the components of the formulation and transfer the blend to the mold.
In conventional practice, particularly with conventional casting techniques using amine chain extenders, no catalyst is used. In such instances, typical in-mold residence times range from about 1 to about 5 hours, depending on the particular components used and the cure temperature. However, in order to decrease energy usage and increase productivity, it is desirable to provide an elastomer formulation which cures more quickly at a lower mold temperature. A common target for preparing elastomers is a 30 minute residence time at about 100.degree.-130.degree. C.
The problems of selecting a catalyst are further complicated when the chain extender is amine-terminated and the high equivalent weight active hydrogen-containing material is hydroxyl-terminated. Amine groups are generally more reactive with isocyanates than are hydroxls, so their presence tends to shorten the pot-life of the elastomer formulation. In addition, although the aforementioned catalysLs catalyze both the hydroxyl/isocyanate and amine/isocyanate reactions, certain types of these catalysts are more specific for one of these reactions than another. It has thus been difficult to obtain optimal properties from elastomers prepared using these catalysts.
Accordingly, it would be desirable to provide a polyurethane-urea formulation which has a suitable pot-life, cures rapidly to a demoldable state, exhibits minimal degraditive oxidation and which provides an elastomer having desirable physical properties.